Video image capture using a digital camera has become very prevalent. Video capture may be used for such applications as video conferencing, video editing, and distributed video training. Many components of hardware and software need to work seamlessly to both get the video data (also referred to as a video “stream”) from the hardware through the various layers of software on the personal computer (PC) and made visible on a display medium such as a monitor.
Producing high quality images in color is desirable for electronic still image capture and video systems. Conventional methods of sensing and producing an image in color include the use of electronic charged-coupled device (CCD) or complimentary metal oxide semiconductor (CMOS) sensor arrays exposed to incident light. The sensor array is an array of photosites or pixels, each photosite loosely defined as a region containing photodetecting circuitry that includes, for example, photodiodes or photogates and associated processing circuitry. In some cases, a color filter array (CFA) is pasted or otherwise processed over the array of photodetecting circuits, such that each photodetecting circuit is covered by a bandpass optical filter that lets light of a particular color pass into and thus be detected by the corresponding circuit. Sensor arrays typically have two or more colors distributed evenly, or according to other schemes, in the array of photosites, where a group of photosites may be assigned a particular color.
To obtain sharp color images from such sensor arrays over a broad range of illuminants, it may be desirable to control the amount of light energy that is incident on the sensor array. One technique for doing so is to modulate the incident light using a physical shutter having variable timing, as in a conventional film camera. The idea of a shutter has been applied to digital solid state cameras in the form of an electronic shutter. For example, in CMOS sensor arrays, the electronic shutter is typically a transistor that couples a photo-detecting element, such as a photodiode, to a charge storage element, such as a capacitor, in each photosite. The electronic shutter transistor operates as a switch in response to a shutter control signal that specifies the “exposure time” by defining the time interval during which the shutter transistor is turned on allowing charge to transfer from the exposed photodiode and accumulate in the capacitor. Alternatively, the shutter transistor may be used to drain pre-stored charge from the capacitor.
Typically, prior art systems that use such electronic shutters provide a single shutter control signal for the entire sensor array. Thus, in most prior art systems, photosites of different colors receive the same exposure time. Although some systems permit different exposure times for each color, they do so by requiring separate and sequential exposures for each color, typically by using a mechanical apparatus to change the color filter over a panchromatic sensor array. Taking sequential exposures for each color effectively precludes taking color pictures of scenes that include motion. Moreover, such a complicated mechanical apparatus, normally used in devices such as flatbed scanners, would present reliability problems and would not be easily adaptable to portable image capture systems such as a digital handheld camera.
Although the technique of using a single shutter control line for the entire sensor array is relatively simple to implement, such an application may also yield poor quality images for illuminants other than broad daylight. Under low or uneven light conditions, colors for which there is a dearth of illumination are less apparent in the resulting image due to increased noise levels in the analog signals received from the photosites tuned for those “weaker” colors. Image processing algorithms may be used to somewhat improve image quality by canceling out the increased noise in the signals for the weaker colors. But such a software solution may introduce undesirable delays before the final image can be viewed. Moreover, the results of such software corrections are not always predictable or consistent.
Therefore, a hardware approach may be desirable to yield more consistent and predictable noise levels. Also, it is desirable to realize a color image system that can somewhat compensate for manufacturing variations in the myriad of photosites in a sensor array, including variations in the photodetecting circuitry and the bandpass filters of the CFA.